1. Field of the Invention
The present invention relates to a voice coding apparatus used for a high efficiency coding of the voice, etc.
2. Description of the Related Art
In the voice coding apparatus, when the voice signal is coded at a low bit rate, the original voice must be regenerated at the regeneration side without losing its essential nature, when heard.
As one means achieving a high efficiency coding the pitch extraction means described as follows is known. That is, the voice waveform for N pitches is sampled from the voice signal, a voice waveform corresponding to one pitch is formed from the voice waveform for these N pitches, and this waveform is coded and transmitted to the receiving side, At the receiving side, the received signal is decoded, and thereafter, is repeated N times, whereby a voice signal for N pitches is generated. Accordingly, transmission bit rate can be reduced by 1/N, compared with the case when the whole voice waveform is transmitted.
In another known means for achieving a high efficiency coding, the band of the voice signal is restricted, to decrease the sampling frequency, and thus the low bit rate is realized. Namely, the band of the voice signal is decreased to 1/M, and is down sampled by a 1/M sampling frequency, whereby the transmission bit rate is decreased to 1/M, compared to the case where the band is not restricted.
The first pitch extracting method for forming a waveform of one pitch from the waveform of a plurality of pitches is disadvantageous in that the coding delay .tau. becomes too long when the voice frequency is low. Namely, when the pitch period is designated as T, and the number of sampled waveforms of the original waveform for the plurality of pitch waveforms which extracts the waveform of one pitch is N, the coding delay .tau. in the transmission side usually becomes EQU .tau.=2N.multidot.T
Assuming that the maximum value T.sub.max of the pitch period is 20 msec and the number of sampled waveforms is N=6, the maximum coding delay .tau..sub.max becomes 240 msec, and this delay causes practical problems in communication. Therefore, the amount of the number of the sampled waveforms N is restricted by the maximum pitch period, but in this case a sufficiently low bit rate cannot be realized.
The second method for restricting the band of the voice signal is disadvantageous in that, when the band restricted voice signal is regenerated at the receiving side, the voice signal is not clear when heard.
Further, in such a voice coding apparatus, to increase the efficiency, an estimate of a pitch period of the voice is sometimes required, and various pitch extraction methods have been proposed for thus purpose.
When the signal is formed by repeating the same waveforms as a voice signal, if the pitch period thereof is assumed to be T, the periods 2T, 3T, 4T, . . . which are multiple of T, also have one period. Accordingly, these multiple pitch periods may be incorrectly detected as voice pitch periods. Especially, such an incorrect extraction may occur when the pitch period T is not a multiple of the sampling period.
To avoid such an incorrect extraction of the pitch period, when the pitch period is a multiple of the sampling period, a true pitch period T is detected as follows. First, the virtual pitch period T(d) is detected, and to detect that this pitch period T(d) is a time of the true pitch period T, it is determined whether or not the period function of one by integer numbers of the pitch period T(d) exists by using an auto-correction function, etc., whereby T(d)/T is determined and the true pitch period T can be extracted.
On the other hand, when the pitch period is not multiple of the sampling period, the above-mentioned method can not be used, and a method of determining a multiple pitch number T(d)/T is not known.